Recently, automated analyzers for performing chemical, biological and biochemical assays have become widespread for use by diagnostic & research laboratories for the rapid and reliable detection of analytes in a variety of biological samples. Analyzers are routinely used to perform a wide variety of assays, most of which involve immunoassays where the high affinity and selectivity of an antibody for its antigen is exploited. Many of these systems are based on measurement of emitted light such as chemiluminescence caused by reactions in the assay.
For example, in many instances, it is desirable to determine the presence and the amount of a specific material in solution (the ‘medium’). Surface-based assays rely on the interaction of the material to be assayed (the ‘analyte’) with a surface that results in a detectable change in any measurable property. For the purpose of this patent application, the term ‘analyte’ refers to the material to be assayed. Examples of analytes include: an ion; a small molecule; a large molecule or a collection of large molecules such as a protein or DNA; a cell or a collection of cells; an organism such as a bacterium or virus. ‘Analyte-specific receptor, or ‘recognition element’ refers to that complementary element that will preferentially bind its partner analyte. This could include: a molecule or collection of molecules; a biomolecule or collection of biomolecules, such as a protein or DNA; a groove on the substrate that has the complementary geometry and/or interaction. In general, in order to assay for a specific analyte, the surface is modified so as to offer the appropriate chemical interaction.
In immunoassays, for example, one takes advantage of the specificity of the antibody-antigen interaction: A surface can be coated with an antigen in order to assay for the presence of its corresponding antibody in the solution or vice versa. Similarly, a strand of deoxyribonucleic acid (DNA) can be attached to a substrate and used to detect the presence of its complementary strand in solution. In any of these cases, the occurrence of binding of the analyte to its recognition element on the surface, which thus identifies the presence of the specific analyte in solution, is accompanied by a detectable change. For example, the binding can produce a change in the index of refraction at the interfacial layer; this can be detected by ellipsometry or surface plasmon resonance. Alternatively, the bound analyte molecules may emit light; this emission can be collected and detected, as is the case for fluorescence-based sensors. Non-optical signals may also be used, as in the case of radio immunoassays and acoustic wave sensing devices.
Diffraction is a phenomenon that occurs due to the wave nature of light. When light hits an edge or passes through a small aperture, it is scattered in different directions. But light waves can interfere to add (constructively) and subtract (destructively) from each other, so that if light hits a non-random pattern of obstacles, the subsequent constructive and destructive interference will result in a clear and distinct diffraction pattern. A specific example is that of a diffraction grating, which is of uniformly spaced lines, typically prepared by ruling straight, parallel grooves on a surface. Light incident on such a surface produces a pattern of evenly spaced spots of high light intensity. This is called Bragg scattering, and the distance between spots (or ‘Bragg scattering peaks’) is a unique function of the diffraction pattern and the wavelength of the light source. There is a unique correspondence between a pattern and its diffraction image, although in practice, diffraction is best illustrated by using periodic patterns, because these yield easily recognized diffraction images of clearly defined regions of high and low light intensity.
There is therefore a need for an analyzer which is based on diffraction of light that that offers ease of use, minimal sample handling, low consumable cost and assay versatility in a compact instrument.